1. Field of the Invention
The present invention concerns an arrangement and a method to transmit magnetic resonance signals that are received with the use of local coils.
2. Description of the Prior Art
An arrangement to transmit magnetic resonance signals that are received with the use of local coils is described in the DE 10 2008 023 467.2.
Modern magnetic resonance systems simultaneously receive magnetic resonance signals via local antennas that are placed on the patient. These local antennas are components of what is known as a local coil or “local coil arrangement”. The received magnetic resonance signals are preamplified, conducted out of a central region of the magnetic resonance system and supplied to a shielded receiver in order to be used there for image processing.
The connection lines used for transmission are generally directed within a patient bed supported such that it is movable, and therefore are multiple meters in length. However, the technical problems shown in the following result due to the use of connection lines.
Cables that are used exhibit attenuations in the frequency range of the magnetic resonance signals that are to be transmitted that must be taken into account. A patient-proximal preamplification of the magnetic resonance signals ensues before the transmission for thus purpose, wherein heat is however formed in proximity to the patient. Low-attenuation cables are presently used for transmission that, however, exhibit a large cable diameter and are correspondingly unwieldy.
Each local antenna forms a reception channel that has a connection cable associated with it to transmit the magnetic resonance signal. An unwieldy cable bundle is likewise formed by the plurality of reception channels, which cable bundle is also exposed to increased mechanical loads at the employed cable plugs due to the movement of the patient bed.
The problem of the aforementioned thick cable bundle is exacerbated because the cable bundle runs in part in the radio-frequency field of the magnetic resonance transmission antenna and therefore contains sheath wave barriers. The cable bundle thickness is additionally increased again due to their dimensions.
Conventionally used coaxial multiconductor plugs are expensive in terms of wiring and unwieldy in application for the operator.
Conventionally used preamplifiers must exhibit a high linearity of the characteristic amplifier line over a large range in order to form low-distortion output signals. This is connected with a high power requirement and with significant heating near the patient.
Conventionally, broadband receivers are used that can receive outgoing magnetic resonance signals not only of hydrogen but also of other atoms (also called X-nucleus operation, thus operation with different nuclei). Such receivers have a transmission network that contains ferrite cores. Due to the ferrite cores, the receiver cannot be operated in the immediate environment of the static basic field of the magnetic resonance apparatus, such that again long connection paths are necessary that lead to the technical problems cited above.
In an arrangement to transmit magnetic resonance signals received with the use of local coils as described in DE 10 2008 023 467.2, two local coil signals in a magnetic resonance system are multiplexed via a connection line (frequency multiplexing). The two intermediate frequencies are selected so that they come to lie symmetrically around the scanning frequency of the following analog-digital converter.
Magnetic resonance signals received from local coils are present at a frequency converter and are respectively shifted by the frequency converter to an intermediate frequency and output with this intermediate frequency, wherein the level of the intermediate frequency respectively depends on the frequency of an oscillator signal, which oscillator signal is present at the frequency converter. The oscillator signal is respectively supplied to the local coil arrangement via coaxial cables, which coaxial cables are likewise used to transmit magnetic resonance signals (with an intermediate frequency) to an evaluation device. One or more oscillator signals (to the local coil arrangement) and one or more magnetic resonance signals (respectively transformed to an intermediate frequency, traveling away from the local coil arrangement) can thus be transferred on one coaxial cable.
A problem occurs when the arrangement should be used for the reception of X-nucleus signals (signals of different excited atomic nuclei, for example 19F, 3He, 31P, 7Li, 129Xe, 23Na, 13C, 17O, 15N etc.) which lie markedly below RF1H in the frequency spectrum. In these cases at least one of the two LO frequencies (respectively generated by an oscillator) moves near to the intermediate frequency band ZF1 (used to transmit usable signals from local coils) such that a spectral ZF/LO separation in the corresponding filters is complicated. The LO and intermediate frequencies are associated with the respective X-nuclei in the table “X-nuclei LO frequency plan). LO frequencies lower than 30 MHz are hereby not used for the cited reason.